Nitrogen fixation is one of the most important biological processes, because it is the main gateway of nitrogen atom in biosphere. Biological nitrogen fixation is catalyzed by nitrogen enzyme which reduce nitrogen gas to ammonium ion with hydrolysis of ATP.Despite the wealth of knowledge on structure, enzymology and synthesis of nitrogenase itself, little is known about how reducing power is supplied to the enzyme except in the genetically well studied bacterium Klebsiella pneumonia. We have studied the electron transport pathway to nitrogenase in the purple photosynthetic bacterium Rhodobacter capsulatus which is taxonomically close to rhizobia. First, we have site-specifically engineered R.capsulatus ferredoxin I that is the primary electron donor to nitrogenase. With series of engineered genes and a purified products, the unique structural feature of this group of ferredoxins were related to their extremely low redox potential. Second, we analyzed the R.capsulatus rnf genes that are essential for nitrogen fixation under illuminated conditions. RnfA protein was shown to span the chromatophore membrane with its odd-numbered hydrophilic regions exposed to periplasm, whereas RnfB and RnfC proteins were revealed to situate at the periphery of the chromatophore membranes. The contents in cellular fractions indicated that the three proteins stabilize each other, supporting a hypothesis that the Rnf products are subunits of a membrane complex. Finally, we detected homologs of rnf genes in Haemophilus influenzae, Vibrio alginolyticus and E.coli. Close comparisons revealed that RnfC has potential binding sites for NADH and FMN which are similar to those found in proton-translocating NADH-quinone oxidoreductases and that RnfA,RnfD and RnfE show similarity to subunits of sodium-translocating NADH-quinone oxidoreductases. We predict that the putative Rnf complex represents a novel family of energy-coupling NADH-oxidoreductases.